The central objective of the proposed research is to further elucidate the mechanisms of action of intracellular messengers such as calcium ion, c-AMP and c-GMP on electrical properties of photoreceptor cell membranes, in particular the membrane conductances. Experiments will be done on Limulus ventral photoreceptor, utilizing voltage clamp in conjunction with application and injection methods. In parallel with these studies a radioimmune assay will enable us to measure directly, changes in cytosol levels of cyclic nucleotides (c-GMP and c-AMP) that are induced by changes in extra-cellular agents and light intensity. A rapid assay will be used to determine the kinetics of the light-induced changes in cyclic nucleotides and determine whether it is rapid enough to be involved in transduction. These experiments are intended to open the way for approaches which will lead to understanding the roles Ca, and cyclic nucleotides play in the phototransduction process, with particular emphasis on the modulation of these regulatory agents on the electrical properties of photoreceptors.